The present invention relates generally to the field of telemetry systems for transmitting information through a flowing stream of fluid. More particularly, the invention relates to the field of mud pulse telemetry where information detected at the bottom of a well bore is transmitted to the surface by means of pressure pulses created in the mud stream that is circulating through the drill string. Still more particularly, the invention relates to a surface detector for amplifying the signal transmitted by the pressure pulses during MWD or other drilling operations, and for providing an improved signal-to-noise ratio as compared to conventional mud pulse telemetry means.
Drilling oil and gas wells is carried out by means of a string of drill pipes connected together so as to form a drill string. Connected to the lower end of the drill string is a drill bit. The bit is rotated and drilling accomplished by either rotating the drill string, or by use of a downhole motor near the drill bit, or by both methods. Drilling fluid, termed mud, is pumped down through the drill string at high pressures and volumes (such as 3000 p.s.i. at flow rates of up to 1400 gallons per minute) to emerge through nozzles or jets in the drill bit. The mud then travels back up the hole via the annulus formed between the exterior of the drill string and the wall of the borehole. On the surface, the drilling mud is cleaned and then recirculated. The drilling mud is used to cool the drill bit, to carry chippings from the base of the bore to the surface, and to balance the hydrostatic pressure in the rock formations.
When oil wells or other boreholes are being drilled, it is frequently necessary or desirable to determine the direction and inclination of the drill bit and downhole motor so that the assembly can be steered in the correct direction. Additionally, information may be required concerning the nature of the strata being drilled, such as the formation's resistivity, porosity, density and its measure of gamma radiation. It is also frequently desirable to know other down hole parameters, such as the temperature and the pressure at the base of the borehole, as examples. Once these data are gathered at the bottom of the bore hole, it is typically transmitted to the surface for use and analysis by the driller.
One prior art method of obtaining at the surface the data taken at the bottom of the borehole is to withdraw the drill string from the hole, and to lower the appropriate instrumentation down the hole by means of a wire cable. Using such "wireline" apparatus, the relevant data may be transmitted to the surface via communication wires or cables that are lowered with the instrumentation. Alternatively, the instrumentation may include an electronic memory such that the relevant information may be encoded in the memory to be read when the instrumentation is subsequently raised to the surface. Among the disadvantages of these wireline methods are the considerable time, effort and expense involved in withdrawing and replacing the drill string, which may be, for example, many thousands of feet in length. Furthermore, updated information on the drilling parameters is not available while drilling is in progress by wireline techniques.
A much-favored alternative is to employ sensors or transducers positioned at the lower end of the drill string which, while drilling is in progress, continuously or intermittently monitor predetermined drilling parameters and formation data and transmit the information to a surface detector by some form of telemetry. Such techniques are termed "measurement while drilling" or MWD. MWD results in a major savings in drilling time and cost compared to the wireline methods described above.
Typically, the down hole sensors employed in MWD applications are positioned in a cylindrical drill collar that is positioned close to the drill bit. The MWD system then employs a system of telemetry in which the data acquired by the sensors is transmitted to a receiver located on the surface. There are a number of telemetry systems in the prior art which seek to transmit information regarding downhole parameters up to the surface without requiring the use of a wireline tool. Of these, the mud pulse system is one of the most widely used telemetry systems for MWD applications.
The mud pulse system of telemetry creates acoustic signals in the drilling fluid that is circulated under pressure through the drill string during drilling operations. The information that is acquired by the downhole sensors is transmitted by suitably timing the formation of pressure pulses in the mud stream. The information is received and decoded by a pressure transducer and computer at the surface.
In a mud pressure pulse system, the drilling mud pressure in the drill string is modulated by means of a valve and control mechanism, generally termed a pulser or mud pulser. The pulser is usually mounted in a specially adapted drill collar positioned above the drill bit. The generated pressure pulse travels up the mud column inside the drill string at the velocity of sound in the mud. Depending on the type of drilling fluid used, the velocity may vary between approximately 3000 and 5000 feet per second. The rate of transmission of data, however, is relatively slow due to pulse spreading, modulation rate limitations, and other disruptive forces, such as the ambient noise in the drill string. A typical pulse rate is on the order of a pulse per second. Some present day systems operate at higher frequencies, for example at 8-12 pulses per second. Representative examples of mud pulse telemetry systems may be found in U.S. Pat. Nos. 3,949,354, 3,958,217, 4,216,536, 4,401,134, and 4,515,225.
Mud pressure pulses can be generated by opening and closing a valve near the bottom of the drill string so as to momentarily restrict the mud flow. In a number of known MWD tools, a "negative" pressure pulse is created in the fluid by temporarily opening a valve in the drill collar so that some of the drilling fluid will bypass the bit, the open valve allowing direct communication between the high pressure fluid inside the drill string and the fluid at lower pressure returning to the surface via the exterior of the string.
Alternatively, a "positive" pressure pulse can be created by temporarily restricting the downwardly flow of drilling fluid by partially blocking the fluid path in the drillstring. One type of positive pulser is the mud siren. The mud siren includes a rotating member which includes apertures which periodically restrict the mud flow in the drill string. This produces a train of pulses which are phase modulated to transmit data.
Whatever type of pulse system is employed, detection of the pulses at the surface is sometimes difficult due to attenuation of the signal and the presence of noise generated by the mud pumps, the downhole mud motor and elsewhere in the drilling system. Typically, a pressure transducer is mounted directly on the line or pipe that is used to supply the drilling fluid to the drill string. An access port or tapping is formed in the pipe, and the transducer is threaded into the port. With some types of transducers, a portion of the device extends into the stream of flowing mud where it is subject to wear and damage as a result of the abrasive nature and high velocity of the drilling fluid. In any case, the transducer detects variations in the drilling mud pressure at the surface and generates electrical signals responsive to these pressure variations.
Unfortunately, the pressure pulses at the surface may frequently be weak and therefore difficult to detect or to distinguish from background noise. Because of the substantial noise created by the mud pumps and other system components, the signal-to-noise ratio is often very low. Such low signal-to-noise ratios may be increased by increasing the strength of the downhole signal that is generated by the mud pulser. This may be accomplished, for example, by altering the distance between various components which make up the valves and flow restricters in the pulser. While these alterations can increase signal strength, they are often undesirable since the likelihood of erosion and jamming of the valve components increases due to debris in the mud stream. Another means to improve signal detection is to employ special signal conditioning techniques in order to extract the desired signal from the background noise. This alternative, however, necessitates the use of sophisticated and expensive electronic signal processing equipment. Even using such equipment, however, detection can still be unreliable or impossible in certain circumstances.
Thus, due to the drilling industry's ever increasing reliance on MWD techniques, and due to the present inadequacies with respect to detecting a mud pulse signals, there remains a need in the at for a detector that is capable of enhancing the amplitude of the acoustic signal seen by the pressure transducer. Preferably, such a detector would be relatively inexpensive and simple to construct. Due to the substantial number of existing detection systems now in use, it would be advantageous if the detector could be constructed, at least in part, from the components presently in use. Preferably, the detector would permit the transducer to be positioned outside the mud flow path such that it would not be susceptible to abrasive damage from the flowing drilling fluid. It would be ideal if the detector would also provide for an increased signal-to-noise ratio in addition to the increase in signal amplitude.